Lessons Learned pushing a stock bottom and "stock" valvetrain
02-22-2016, 07:20 AM
#1
Lessons Learned pushing a stock bottom and "stock" valvetrain Here's a copy of what I posted on the ImpalaSSforum:
"After a (what most would consider successful) 6-yr run on pushing 500+ hp NA and 600+ hp on spray with a stock bottom HR cam build, here are some ramblings which may help some others who plan on pushing the limits with stock parts and a hydraulic valvetrain.
For those who don't know, my little "tree fiddy" finally breathed its last at the ImpalaSS Nationals last July at BG, KY.
Thanks to a topnotch rebuild of the bottom end by Mike Harris and his local machine shop, I didn't experience the typical "spun rod bearing" failure that most stock bottoms succumb to. It appears that the weak point of the stock rotating assembly is not necessarily the rods, but maybe the pistons, even when you take care of them via careful tuning to avoid any detonation. (There were zero signs of detonation in the motor).
The #6 piston appears to have broken in the area of the lower ring groove, which led to expansion of the piston diameter, sudden added drag in the bore, and subsequent failure of the rod just below the pin. I have to attribute the piston failure to the several million cycles in the 6500-7000 rpm range I've put on them with the last couple of seasons of open track roadcourse usage, as opposed to only thousands of cycles on the dragstip in that rpm range. Drag race only, the motor could have lasted another 6 years!
So, that's a discussion of the rods and pistons. Regarding the crank, it's fine except for the trash that got drug into the mains during the carnage. We did see quite a bit of main cap (stock 2-bolt mains) walkage though, and I don't know if that helps to explain the main bearing wear being most prevalent on the outer edges of the bearings. If that's not the reason, then I'd have to say the crank had been flexing quite a bit between bearing journals.
Now for the valvetrain.
Although I'm not complaining, since I got some good service out of the setup, the autopsy shows I was using the AI-specified setup beyond its capabilities. In short, I would not advise using stock hydraulic lifters to 7000 rpm with an aggressive cam as I've been doing. About 12 of the 16 valves have tagged the pistons, and the cam lobes show that the lifter rollers did not maintain contact around the lobe profile. And I was running 160 lbs on the seat with the bigblock style Manley beehive springs.
So, this time around, naturally the motor is going to get some nice light forged pistons and rods, and a high quality forged crank, most likely 3.875" stroke.
And for the valvetrain, for what I've been doing, pushing extreme rpm for hours per season rather than minutes, I'd have to recommend either short-travel hydraulic lifters, or better yet, a solid roller setup. And since I will continue to use the motor for open track events, I think I'm going to go with a Low Lash Solid Roller setup on the next engine build, which should give me some added durability over the standard lash solid roller cams which are perfectly fine for dragrace motors that don't see the high time at those rpm's.
After seeing what's been going on with the valvetrain, I think I've been giving up some significant power with collapsing lifters/loss of valve lift and floating/loss of control of the valves."
"After a (what most would consider successful) 6-yr run on pushing 500+ hp NA and 600+ hp on spray with a stock bottom HR cam build, here are some ramblings which may help some others who plan on pushing the limits with stock parts and a hydraulic valvetrain.
For those who don't know, my little "tree fiddy" finally breathed its last at the ImpalaSS Nationals last July at BG, KY.
Thanks to a topnotch rebuild of the bottom end by Mike Harris and his local machine shop, I didn't experience the typical "spun rod bearing" failure that most stock bottoms succumb to. It appears that the weak point of the stock rotating assembly is not necessarily the rods, but maybe the pistons, even when you take care of them via careful tuning to avoid any detonation. (There were zero signs of detonation in the motor).
The #6 piston appears to have broken in the area of the lower ring groove, which led to expansion of the piston diameter, sudden added drag in the bore, and subsequent failure of the rod just below the pin. I have to attribute the piston failure to the several million cycles in the 6500-7000 rpm range I've put on them with the last couple of seasons of open track roadcourse usage, as opposed to only thousands of cycles on the dragstip in that rpm range. Drag race only, the motor could have lasted another 6 years!
So, that's a discussion of the rods and pistons. Regarding the crank, it's fine except for the trash that got drug into the mains during the carnage. We did see quite a bit of main cap (stock 2-bolt mains) walkage though, and I don't know if that helps to explain the main bearing wear being most prevalent on the outer edges of the bearings. If that's not the reason, then I'd have to say the crank had been flexing quite a bit between bearing journals.
Now for the valvetrain.
Although I'm not complaining, since I got some good service out of the setup, the autopsy shows I was using the AI-specified setup beyond its capabilities. In short, I would not advise using stock hydraulic lifters to 7000 rpm with an aggressive cam as I've been doing. About 12 of the 16 valves have tagged the pistons, and the cam lobes show that the lifter rollers did not maintain contact around the lobe profile. And I was running 160 lbs on the seat with the bigblock style Manley beehive springs.
So, this time around, naturally the motor is going to get some nice light forged pistons and rods, and a high quality forged crank, most likely 3.875" stroke.
And for the valvetrain, for what I've been doing, pushing extreme rpm for hours per season rather than minutes, I'd have to recommend either short-travel hydraulic lifters, or better yet, a solid roller setup. And since I will continue to use the motor for open track events, I think I'm going to go with a Low Lash Solid Roller setup on the next engine build, which should give me some added durability over the standard lash solid roller cams which are perfectly fine for dragrace motors that don't see the high time at those rpm's.
After seeing what's been going on with the valvetrain, I think I've been giving up some significant power with collapsing lifters/loss of valve lift and floating/loss of control of the valves."
02-22-2016, 10:07 AM
#4
11 Second Club
How long have you had the heads and cam setup on the car?
02-22-2016, 03:50 PM
#6
TECH Resident
That's some good info.
I'm going to be going with AI ported stock heads soon(most likely 190's).
To try and get the most of the setup, I very well may also get one of their cams. Can you reveal the lift/duration of your cam? I don't plan on pushing to 7K, but the stock bottom end will see 6300 on a regular basis...and touching 6500 on occasion. So it would be nice to know what kind of cam numbers were giving you the valve float.
I'm going to be going with AI ported stock heads soon(most likely 190's).
To try and get the most of the setup, I very well may also get one of their cams. Can you reveal the lift/duration of your cam? I don't plan on pushing to 7K, but the stock bottom end will see 6300 on a regular basis...and touching 6500 on occasion. So it would be nice to know what kind of cam numbers were giving you the valve float.
02-22-2016, 05:50 PM
#7
That's some good info.
I'm going to be going with AI ported stock heads soon(most likely 190's).
To try and get the most of the setup, I very well may also get one of their cams. Can you reveal the lift/duration of your cam? I don't plan on pushing to 7K, but the stock bottom end will see 6300 on a regular basis...and touching 6500 on occasion. So it would be nice to know what kind of cam numbers were giving you the valve float.
I'm going to be going with AI ported stock heads soon(most likely 190's).
To try and get the most of the setup, I very well may also get one of their cams. Can you reveal the lift/duration of your cam? I don't plan on pushing to 7K, but the stock bottom end will see 6300 on a regular basis...and touching 6500 on occasion. So it would be nice to know what kind of cam numbers were giving you the valve float.
It peaked at 6400 rpm, so if you don't intend to rev past 6500, I'd step back a size to put the powerband more where you will use both sides of it; it makes for a quicker car. You'll be ok with factory style HR lifters, just make sure you're at least 150 lbs on the seat, even with one of their milder cams.
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02-22-2016, 05:54 PM
#8
Oh wow....
You were pushing that engine to 7k!? Bowtienut, you're definitely a nut!
You must of had a perfect tune. I think you got your money's worth out that engine for 6 years and your have a very impressive track record on your sig.
The new engine sound like it will be a beast too.
You were pushing that engine to 7k!? Bowtienut, you're definitely a nut!
You must of had a perfect tune. I think you got your money's worth out that engine for 6 years and your have a very impressive track record on your sig.
The new engine sound like it will be a beast too.
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Westcoastlou (03-19-2024)
02-22-2016, 06:33 PM
#9
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Just wondering if you are going to use the same guy and shop for the new motor? No big deal if you would rather not say.
I think I lost a head gasket on mine last fall and have been looking for a reputable machine shop and possibly builder in the midwest. I would send it to Ellwein but the shipping to the east coast and back is a little rough.
02-22-2016, 07:37 PM
#11
11 Second Club
02-22-2016, 08:19 PM
#12
miles on the block before the build?....about 80k. Cyllinders were honed, new rings, rods resized with ARP bolts. I put probably less than 2000 miles on it after the build, but all of it was racing. It didn't have 100 easy miles on it.
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Westcoastlou (03-19-2024)
02-23-2016, 05:13 AM
#13
I'm curious if like you said it was mostly the autocross/road racing that killed it, or how much the nitrous helped kill it. If you hadn't used nitrous and pushed the pistons that hard and gone over the magical 450ish mark if they would have held up longer. Random guess, how many bottles or shots or passes did you spray through it? When you did autocross and road course, did you hold gears out and not let it shift at all? Like hold 6000-7000 unloaded for a while where drag racing the petal is treated like an on/off button. Do you think the piston to valve touch was just a tiny bit, and 0.026 vs your 0.015 gaskets would have kept it off? This kind of goes back to the totally off topic convo we all had with kingtal0n on the limit of the LTx bottom end being around 450rwhp, being the pistons, rods, 2 bolts and lifters, all of them were hating life, but did a damn good job for a while.
Are your heads trashed?
On your new build have you considered those new Johnson short travel lifters the LS guys have been using? Or have you considered, dare I say it, going to 6 bolt mains and abandoning the red headed step child?
Are your heads trashed?
On your new build have you considered those new Johnson short travel lifters the LS guys have been using? Or have you considered, dare I say it, going to 6 bolt mains and abandoning the red headed step child?
Last edited by bufmatmuslepants; 02-23-2016 at 05:26 AM.
02-23-2016, 07:38 AM
#14
No LS! Autocross: nah, that’s benign, no more than dragracing. A lot of feathering the throttle there, but not at the higher rpms.
Nitrous: I don’t believe so. I’d say about 50 passes total, starting with tuning on the 50, 75, then 100 shot. Piston failure wasn’t up in the area where I’d expect the added cylinder pressure to have an effect. It had been nearly 3 years since I sprayed it.
The roadrace duty is definitely what did it in. I shifted around 6800 on the roadcourses, but yes there are those times when I stayed in it to avoid a shift just before a braking zone and buzzed it to 7000-7100. But no, under those conditions, no feathering of the throttle. It was full on then off. Anyway, this talk you hear of adding rod tensile load under high vac /throttle liftoff is grossly exaggerated. Worst it can EVER be is 14.7 psi x 4 x pi / 4 = 46 lbs additional pull on the rod & piston.
Oh yes, the valve touch was definitely just a “tiny bit”…otherwise I’d see some actual damage! All 8 of the intakes touched in the radius of the relief, not at the bottom of the pocket. I find it hard to believe the 2.00” intakes vs. 1.94” stock would cause that? The exhausts (5 of them I think) touched at the top edge of the relief pocket where it meets the narrow flat at the center of the piston. I’m pretty sure AI didn’t angle mill the heads to get down to 52cc chambers. No way I can blame the tight quench for the PTV contact. The valves were simply out of control. Probably compressing the lifters enough during valve opening that there was lash in the valvetrain where the roller wasn’t following the back side of the lobe and sitting down without bounce. I wouldn’t be surprised if all the visible contact happened that last day at our roadrace competition on the Corvette track in BG at our Nationals. Tired old springs, braking later than ever on the front straight trying to beat my buddy Doc’s time……which I did
With the cylinder hone and stock pistons, I was running the high side of piston-to-bore clearance. With the inertia of those relatively heavy stock pistons, I'm wondering if the rocking and resulting wall contact loads were enough to initiate fatigue cracking in the corners of the lower ring grooves where that #6 failed ???
Heads are unscathed. I’ll be using them on the new build.
Staying LT1, splayed main caps, probably a Callies Compstar crank and rods. Some dished lightweight SRP or Mahle pistons to use my 52cc heads and run pump gas. I’m going tight lash solid. I don’t see much point in the short travel hydraulics for my use. 90% roadcourse work, prob 1000 street miles per year for the foreseeable future. TLSR cams I’m considering require only 165-175 seat load, ramps little to no more radical than what I’d been running, so I figure with a new set up at 190-200 seat, I can stay with my 7/16 stud rocker setup. I think the valvetrain will be much happier dealing with the ~.010 lash than whatever part of that….what .080”? of the short travel hydraulic that decides to compress.
Nitrous: I don’t believe so. I’d say about 50 passes total, starting with tuning on the 50, 75, then 100 shot. Piston failure wasn’t up in the area where I’d expect the added cylinder pressure to have an effect. It had been nearly 3 years since I sprayed it.
The roadrace duty is definitely what did it in. I shifted around 6800 on the roadcourses, but yes there are those times when I stayed in it to avoid a shift just before a braking zone and buzzed it to 7000-7100. But no, under those conditions, no feathering of the throttle. It was full on then off. Anyway, this talk you hear of adding rod tensile load under high vac /throttle liftoff is grossly exaggerated. Worst it can EVER be is 14.7 psi x 4 x pi / 4 = 46 lbs additional pull on the rod & piston.
Oh yes, the valve touch was definitely just a “tiny bit”…otherwise I’d see some actual damage! All 8 of the intakes touched in the radius of the relief, not at the bottom of the pocket. I find it hard to believe the 2.00” intakes vs. 1.94” stock would cause that? The exhausts (5 of them I think) touched at the top edge of the relief pocket where it meets the narrow flat at the center of the piston. I’m pretty sure AI didn’t angle mill the heads to get down to 52cc chambers. No way I can blame the tight quench for the PTV contact. The valves were simply out of control. Probably compressing the lifters enough during valve opening that there was lash in the valvetrain where the roller wasn’t following the back side of the lobe and sitting down without bounce. I wouldn’t be surprised if all the visible contact happened that last day at our roadrace competition on the Corvette track in BG at our Nationals. Tired old springs, braking later than ever on the front straight trying to beat my buddy Doc’s time……which I did
With the cylinder hone and stock pistons, I was running the high side of piston-to-bore clearance. With the inertia of those relatively heavy stock pistons, I'm wondering if the rocking and resulting wall contact loads were enough to initiate fatigue cracking in the corners of the lower ring grooves where that #6 failed ???
Heads are unscathed. I’ll be using them on the new build.
Staying LT1, splayed main caps, probably a Callies Compstar crank and rods. Some dished lightweight SRP or Mahle pistons to use my 52cc heads and run pump gas. I’m going tight lash solid. I don’t see much point in the short travel hydraulics for my use. 90% roadcourse work, prob 1000 street miles per year for the foreseeable future. TLSR cams I’m considering require only 165-175 seat load, ramps little to no more radical than what I’d been running, so I figure with a new set up at 190-200 seat, I can stay with my 7/16 stud rocker setup. I think the valvetrain will be much happier dealing with the ~.010 lash than whatever part of that….what .080”? of the short travel hydraulic that decides to compress.
Last edited by bowtienut; 02-23-2016 at 08:13 AM.
02-23-2016, 10:08 AM
#16
Something I've wondered since this teardown: I was running only 1/8 turn lifter preload to avoid lifter pumpup and hanging a valve open in the case I did get slack in the valvetrain. I wonder if I'd been better off trusting the valvesprings to keep things together and adjusting the stock lifters near bottoming out so they couldn't compress as much during valve lift. Or would I have damaged them in doing that?? no idea how they would respond to bottoming out the plunger. It appears I was giving up some power with the condition I had. Definitely something to be said for short travel lifters, although I think their travel span alone is still way too much to solve this issue. Maybe their merit is their tighter/slower bleed-down rate rather than their short travel ?
02-24-2016, 05:22 AM
#20
Not going for all-out power by any means. I just want reliability and low maintenance for the track.